Deposition method for coating glass and the like

ABSTRACT

This disclosure describes transparent glass window structures of the type bearing a first coating of infra-red reflective material which is advantageously less than about 0.85 microns in thickness and wherein the observance of iridescence resulting from such a first coating is markedly reduced by provision of a layer of continuously varying refractive index between the glass and the coating, such that the refractive index increases continuously from the glass to the first coating, thereby preventing the observation of iridescence. The invention also encompasses simple processes for providing such windows. A particular advantage of the invention is its efficacy with clear and lightly tinted glasses wherein the problem of iridescent color has had its greatest commercial impact.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 784,542filed Apr. 4, 1977.

BACKGROUND OF THE INVENTION

This invention relates to glass structures bearing a thin, functional,inorganic coating (e.g. a coating of tin oxide forming means to promotereflectivity of infra-red light) which structures have improvedappearance as a consequence of reduced iridescence historicallyassociated with said thin coatings, and methods for achieving theaforesaid structures.

Glass and other transparent materials can be coated with transparentsemiconductor films such as tin oxide, indium oxide or cadium stannate,in order to reflect infra-red radiation. Such materials are useful inproviding windows with enhanced insulating value (lower heat transport),e.g. for use in ovens, architectural windows, etc. Coatings of thesesame materials also conduct electricity, and are employed as resistanceheaters to heat windows in vehicles in order to remove fog or ice.

One objectionable feature of these coated windows is that they showinterference colors (iridescence) in reflected light, and, to a lesserextent, in transmitted light. This iridescence has been a seriousbarrier to widespread use of these coated windows (see, for example,American Institute of Physics Conference Proceeding No. 25, New York,1975, Page 288.)

In some circumstances, i.e. when the glass is quite dark in tone (say,having a light transmittance of less than about 25%) this iridescence ismuted and can be tolerated. However, in most architectural wall andwindow applications, the iridescent effect normally associated withcoatings of less than about 0.75 microns is aesthetically unacceptableto many people (See, for example, U.S. Pat. No. 3,710,074 to Stewart.)

Iridescent colors are quite a general phenomenon in transparent films inthe thickness range of about 0.1 to 1 micron, especially at thicknessesbelow about 0.85 micron. Unfortunately, it is precisely this range ofthickness which is of practical importance in most commercialapplications. Semiconductor coatings thinner than about 0.1 micron donot show interference colors, but such thin coatings have a markedlyinferior reflectance of infra-red light, and a markedly reduced capacityto conduct electricity.

Coatings thicker than about 1 micron also do not show visibleiridescence in daylight illumination, but such thick coatings are muchmore expensive to make, since larger amounts of coating materials arerequired, and the time necessary to deposit the coating iscorrespondingly longer. Furthermore, films thicker than 1 micron have atendency to show haze, which arises from light scattering from surfaceirregularities, which are larger on such films. Also, such films show agreater tendency to crack, under thermal stress, because of differentialthermal expansion.

As a result of these technical and economic constraints, almost allpresent commercial production of such coated glass articles comprisefilms in the thickness range of about 0.1 to 0.3 microns, which displaypronounced iridescent colors. Almost no architectural use of this coatedglass is made at present, despite the fact that it would becost-effective in conserving energy to do so. For example, heat loss byinfra-red radiation through the glass areas of a heated building canapproximate about one-half of the heat loss through uncoated windows.The presence of iridescent colors on these coated glass products is amajor reason for the failure to employ these coatings.

Co-pending application, Ser. No. 784,542, discloses means to reduce thisiridescence to unobservably small values, by means of an additionallayer or layers placed in register with the main coating, including agradient-type coating. The present disclosure is directed primarilytoward improved means for forming such a gradient-type anti-iridescentlayer.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide means to eliminatethe visible iridescence from semi-conducting thin film coatings onglass, while maintaining their desirable properties of visibletransparency, infra-red reflectivity, and electrical conductivity.

Another object of the present invention is to achieve the above goalswithout increasing the cost of production significantly over the cost ofusing ordinary iridescent films.

Another object of the present invention is to achieve the above aimswith a process which is continuous and fully compatible with modernmanufacturing processes in the glass industry.

A further object of the present invention is to achieve all of the abovegoals with products which are highly durable and stable to light,chemicals and mechanical abrasion.

Another object is to achieve all of the above goals using materialswhich are sufficiently abundant and readily available to permitwidespread use.

A further object of the invention is to provide means to reduce thetotal amount of light reflected from the coated surface of glass andthereby increase the total transmission of light by the glass.

Another object of the invention is to provide a glass structurecomprising a compound coating wherein an outer coating is formed of aninfra-red reflecting surface of about 0.7 micron or less and wherein aninner coating forms means for (a) reducing haze on the coated glass and,simultaneously and independently (b) reducing the iridescence of theglass structure.

A further object of the invention is to provide a glass structure havingthe non-iridescent characteristics referred to above which structure ischaracterized by a gradual change in coating composition between glassand said outer coating.

Other objects of the invention are to provide novel apparatus andprocesses which are suitable for making the above identified novelproducts and, indeed, which are suitable for use in making coatings ofgradually changing compositions and from gaseous reactants whether ornot such coating be on glass or some other substrate and whether or notsuch coatings comprise a maximum amount of one component within or at anextremity of the depth of the coating structure.

Other objects of the invention will be obvious to those skilled in theart on reading the instant invention.

The invention utilizes the formation of layers of transparent materialbetween the glass and the semiconductor film. These layers haverefractive indices intermediate between those of the glass and thesemi-conductor film. With suitable choices of thickness and refractiveindex values of these intermediate layers, it has been discovered thatthe iridescent colors can be made too faint for most human observers todetect, and certainly too faint to interfere with widespread commercialuse even in architectural applications. Suitable materials for theseintermediate layers are also disclosed herein, as well as processes forthe formation of these layers.

In the preferred form of the invention, these intermediate layers blendtogether continuously to form a graded layer in which the refractiveindex varies, preferably in a smooth transition, as one moves throughthe layer away from the glass toward the semiconductor coating, from avalue at the glass surface matching the index of the glass, to arefractive index value matching that of the overlying semiconductorfilm, at a point proximate to that overlying film.

A coating with refractive index varying through its thickness may beproduced by a novel method disclosed herein, in which a gas mixture withcomponents of different reactivities, flows along the surface of amoving glass substrate.

METHODS AND ASSUMPTIONS

It is believed desirable, because of the subjective nature of colorperception, to provide a discussion of the methods and assumptions whichhave been used to evaluate the invention disclosed herein. It should berealized that the application of much of the theory discussed below isretrospective in nature because the information necessarily is beingprovided in hindsight, i.e. by one having a knowledge of the inventiondisclosed herein.

In order to make a suitable quantitative evaluation of various possibleconstructions which suppress iridescent colors, the intensities of suchcolors were calculated using optical data and color perception data. Inthis discussion, film layers are assumed to be planar, with uniformthickness and uniform refractive index within each layer. In thisevaluation any refractive index changes are taken to be abrupt at theplanar interfaces between adjacent film layers. A continuously varyingrefractive index may be modelled as a sequence of a very large number ofvery thin layers with closely spaced refractive indices. Real refractiveindices are used, corresponding to negligible absorption losses withinthe layers. The reflection coefficients are evaluated for normallyincident plane waves of unpolarized light.

Using the above assumptions, the amplitudes for reflection andtransmission from each interface are calculated from Fresnel's formulae.Then these amplitudes are summed, taking into account the phasedifferences produced by propagation through the relevant layers. Theseresults have been found to be equivalent to the Airy formulae (See, forexample, Optics of Thin Films, by F. Knittl, Wiley and Sons, New York,1976) for multiple reflection and interference in thin films, when thoseformulae were applied to the same cases.

The calculated intensity of reflected light has been observed to varywith wavelength, and thus is enhanced in certain colors more than inothers. To calculate the reflected color seen by an observer, it isdesirable first to specify the spectral distribution of the incidentlight. For the purpose, one may use the International Commission onIllumination Standard Illuminant C, which approximates normal daylightillumination. The spectral distribution of the reflected light is theproduct of the calculated reflection coefficient and the spectrum ofIlluminant C. The color hue and color saturation as seen in reflectionby a human observer, are then calculated from this reflected spectrum,using the uniform color scales such as those known to the art. Oneuseful scale is that disclosed by Hunter in Food Technology, Vol. 21,pages 100-105, 1967. This scale has been used in deriving therelationship now to be disclosed.

The results of calculations, for each combination of refractive indicesand thicknesses of the layers, are a pair of numbers, i.e. "a" and "b"."a" represents red (if positive) or green (if negative) color hue, while"b" describes a yellow (if positive) or blue (if negative) hue. Thesecolor hue results are useful in checking the calculations against theobservable colors of samples including those of the invention. A singlenumber, "c", represents the "color saturation": c=(a² +b²)1/2. Thiscolor saturation index, "c", is directly related to the ability of theeye to detect the troublesome iridescent color hues. When the saturationindex is below a certain value, one is not able to see any color in thereflected light. The numerical value of this threshold saturation ofobservability depends on the particular uniform color scale used, and onthe viewing conditions and level of illumination (see, for example, R.S. Hunter, The Measurement of Appearance, Wiley and Sons, New York,1975, for a review of numerical color scales.)

In order to establish a basis for comparison of structures a firstseries of calculations was carried out to simulate a singlesemiconductor layer on glass. The refractive index of the semiconductorlayer was taken at 2.0, which is a value approximating tin oxide, indiumoxide, or cadmium stannate films. The value 1.52 was used for the glasssubstrate; this is a value typical of commercial window glass. Thecalculated color saturation values are plotted in FIG. 1 as a functionof the semiconductor film thickness. The color saturation is found to behigh for reflections from films in the thickness range 0.1 to 0.5microns. For films thicker than 0.5 micron, the color saturationdecreases with increasing thickness. These results are in accord withqualitative observations of actual films. The pronounced oscillationsare due to the varying sensitivity of the eye to different spectralwavelengths. Each of the peaks corresponds to a particular color, asmarked on the curve (R=red, Y=yellow, G=green, B=blue).

Using these results, the minimum observable value of color saturationwas established by the following experiment: Tin oxide films withcontinuously varying thickness, up to about 1.5 microns, were depositedon glass plates, by the oxidation of tetramethyltin vapor. The thicknessprofile was established by a temperature variation from about 450° C. to500° C. across the glass surface. The thickness profile was thenmeasured by observing the interference fringes under monochromaticlight. When observed under diffused daylight, the films showedinterference colors at the correct positions shown in FIG. 1. Theportions of the films with thicknesses greater than 0.85 micron showedno observable interference colors in diffused daylight. The green peakcalculated to lie at a thickness of 0.88 micron could not been seen.Therefore, the threshold of observability is above 8 of these colorunits. Likewise, the calculated blue peak at 0.03 micron could not beenseen, so the threshold is above 11 color units, the calculated value forthis peak. However, a faint red peak at 0.81 micron could be seen undergood viewing conditions, e.g. using a black velvet background and nocolored objects in the field of view being reflected, so the thresholdis below the 13 color units calculated for this color. We conclude fromthese studies that the threshold for observation of reflected color isbetween 11 and 13 color units on this scale, and therefore we haveadopted a value of 12 units to represent the threshold for observabilityof reflected color under daylight viewing conditions. In other words, acolor saturation of more than 12 units appears as a visibly colorediridescence, while a color saturation of less than 12 units is seen asneutral.

It is believed that there will be little objection to commercializationof products having color saturation values of 13 or below. However, itis much preferred that the value be 12 or below and, as will appear inmore detail hereinafter, there appears to be no practical reason why themost advantageous products according to the invention, e.g. thosecharacterized by wholly color-free surfaces, i.e. below about 8, cannotbe made economically.

A value of 12 or less is indicative of a reflection which does notdistort the color of a reflected image in an observable way. Thisthreshold value of 12 units is taken to be a quantitative standard withwhich one can evaluate the success or failure of various multilayerdesigns, in suppressing the iridescence colors.

Coatings with a thickness of 0.85 micron or greater have colorsaturation values less than this threshold of 12, as may be seen inFIG. 1. Experiments confirm that these thicker coatings do not showobjectionable iridescence colors in daylight illumination.

USE OF AN INTERLAYER OF GRADUATED REFRACTIVE INDEX

It has been discovered that a film intermediate betwen the glasssubstrate and a semiconductor layer can be built up of a gradedcomposition, e.g. gradually changing from a silica film to a tin oxidefilm. Such a film may be pictured as one comprising a very large numberof intermediate layers. Calculations have been made of reflected colorsaturation for a variety of refractive index profiles between glass ofrefractive index n=1.52 and semiconductor coatings of refractive indexn=2.0. For transition layers thicker than about 0.15 micron, thecalculated color saturation index is usually below 12, i.e. neutral tothe eye, and, for, transition layers more than about 0.3 microns thecolor is always undetectable. The exact shape of the refractive indexprofile has very little effect on these results, provided only that thechange is gradual through the graded layer.

WHAT MATERIALS CAN BE USED

A wide range of transparent materials are among those which can beselected to make products meeting the aforesaid criteria by forminganti-iridescent undercoat layers. Various metal oxides and nitrides, andtheir mixtures have the correct optical properties of transparency andrefractive index. Table A lists some mixtures which have the correctrefractive index range between glass and a tin oxide or indium oxidefilm. The weight percents necessary can be taken from measuredrefractive index versus composition curves, or calculated from the usualLorentz-Lorenz law for refractive indices of mixtures (Z. Knittl, Opticsof Thin Films, Wiley and Sons, New York, 976, page 473), using measuredrefractive indices for the pure films. This mixing law generally givessufficiently accurate interpolations for optical work, although thecalculated refractive indices are sometimes slightly lower than themeasured values. Film refractive indices also vary somewhat withdeposition method and conditions employed.

FIG. 3 gives a typical curve of refractive index versus composition forthe important case of silicon dioxide-tin dioxide mixtures.

Table A. Some combinations of compounds yielding transparent mixtureswhose refractive indices span the range from 1.5 to 2.0

SiO₂ and SnO₂

SiO₂ and Si₃ N₄

SiO₂ and TiO₂

SiO₂ and In₂ O₃

SiO₂ and Cd₂ SnO₄

PROCESS FOR FORMING FILMS

Films can be formed by simultaneous vacuum evaporation of theappropriate materials of an appropriate mixture. For coating of largeareas, such a window glass, chemical vapor deposition (CVD) at normalatmospheric pressure is more convenient and less expensive. However, theCVD method requires suitable volatile compounds for forming eachmaterial. Silicon dioxide can be deposited by CVD from gases such assilane, SiH₄, dimethylsilane (CH₃)₂ SiH₂, etc. Liquids which aresufficiently volatile at room temperature are almost as convenient asgases; tetramethylin is such a source for CVD of tin compounds, while(C₂ H₅)₂ SiH₂ and SiCl₄ are volatile liquid sources for silicon.

A continuously graded layer of mixed silicon-tin oxide may be built upduring a continuous CVD coating process on a continuous ribbon of glassby the following novel procedure. A gas mixture is caused to flow in adirection parallel to the glass flow, under (or over) the ribbon of hotglass, as shown, for example, in FIG. 4. The gas mixture contains anoxidizable silicon compound, an oxidizable tin compound, and oxygen orother oxidizing gas. The compounds are chosen so that the siliconcompound is somewhat more quickly oxidized than is the tin compound, sothat the oxide deposited on the glass where the gas mixture firststrikes the hot glass surface, is mainly composed of silicon dioxide,with only a small percentage of tin dioxide. The proportions of siliconand tin compounds in the vapor phase are adjusted so that this intiallydeposited material has a refractive index wich closely matches that ofthe glass itself. Then, as the gas continues in contact with the glasssurface, the proportion of tin oxide in the deposited film increases,until at the exhaust end of the deposition region, the silicon compoundhas been nearly completely depleted in the gas mixture, and the depositformed there is nearly pure tin oxide. Since the glass is alsocontinually advancing from the relatively silicon-rich (initial)deposition region to a relatively tin-rich (final) region, the glassreceives a coating with a graded refractive index varying continuouslythrough the coating thickness, starting at the glass surface with avalue matching that of glass, and ending at its outer surface, with avalue matching that of tin oxide. Subsequent deposition regions,indicated in FIG. 3, can then be used to build up further layers of puretin oxide, or layers of tin oxide doped, for example, with fluorine.

A suitable gas mixture for this purpose, preferably includes theoxidizable silicon compounds, 1,1,2,2, tetramethyldisilane (HMe₂ SiSiMe₂H); 1,1,2, trimethyldisilane H₂ MeSiSiMe₂ H, and/or 1,2, dimethyldislane(H₂ MeSiSiMeH₂) along with tetramethylin (Me₄ Sn). It has been foundthat the initially deposited film is silicon-rich, and has a refractiveindex close to that of glass, while the later part of the deposit isalmost pure tin oxide.

The Si--H bonds in the above-disclosed silicon compounds are highlyuseful in the process, since compounds without Si--H bonds, such astetramethylsilane Me₄ Si, or hexamethyldisilane Me₃ SiSiMe₃, areoxidized more slowly than is tetramethylin, and the initial deposit ismainly tin oxide, and the latter part of the deposit is mainly silicondioxide. In such a case, i.e. when one is using compounds such as Me₄Si, one may flow the gas and glass in opposite directions in order toachieve the desired gradation of refractive index, provided the gas flowis faster than the glass flow. However, the preferred embodiment is touse the more easily oxidizable silicon compounds, and concurrent gas andglass flow directions.

It is also desirable, in forming coatings wherein the composition variesmonotonically with distance from the substrate, that the siliconcompounds have a Si--Si bond as well as the Si--H bond. For example, acompound containing Si--H but not SiSi bonds, dimethylsilane Me₂ SiH₂,along with tetramethylin, produces an initial deposit of nearly pure tinoxide, which then becomes silicon-rich at an intermediate time andfinally becomes tin-rich still later in the deposition. AlthoughApplicant does not wish to be bound by the theory, it is believed thatthe Si--Si--H arrangement facilitates rapid oxidation by an initialthermally induced decomposition in which the hydrogen migrates to theneighboring silicon HMe₂ Si--SiMe₂ H→Me₂ SiH₂ +Me₂ Si. The reactivedimethylsilylene Me₂ Si species is then rapidly oxidized, releasing freeradicals such as hydroxyl (OH), which then rapidly abstract hydrogenfrom the Si--H bonds, thus creating more reactive silylene radicals,forming a chain reaction. The tetramethyltin is less reactive to theseradicals, and thus mainly enters into the later stages of the oxidation.The Me₂ SiH₂ lacks the rapid initial decomposition step, and thus,cannot begin oxidation until after some tetramethyltin has decomposed toform radicals (CH₃, OH, O, etc.) which then preferentially attack theMe₂ SiH₂, at intermediate times, until the Me₂ SiH₂ is consumed, afterwhich stage the oxidation of tetramethyltin becomes dominant again.

It is preferred to have at least two methyl groups in the disilanecompound, since the disilanes with one or no methyl substituents arespontaneously flammable in air, and thus must be pre-mixed with an inertgas such as nitrogen.

Other hydrocarbon radicals, such as ethyl, propyl, etc., may replacemethyl in the above compounds, but the methyl ones are more volatile andare preferred.

Higher partially alkylated polysilanes, such as polyalkyl-substitutedtrisilanes or tetrasilanes, function in a way similar to the disilanes.However, the higher polysilanes are harder to synthesize, and lessvolatile than the disilanes, which are therefore preferred.

When the initial deposition of the silica-tin oxide films contain lessthan about 40% of tin oxide, there will be little or no haze created atthe interface of the glass substrate and the coating thereover. If it,for some reason, is desired to start the gradient above about 30% of tinoxide, it is preferable to have the glass coated with a haze-inhibitinglayer, i.e. silicon dioxide. Such a haze-inhibiting layer may be verythin, e.g. in the nature of 25 to 100 angstroms.

IN THE DRAWINGS

FIG. 1 is a graph illustrating the variation of calculated colorintensity of various colors with semiconductor film thickness.

FIG. 2 illustrates, schematically and in section, a non-iridescentcoated glass constructed according to the invention, with ananti-iridescent interlayer of continuously-varying composition accordingto the invention.

FIG. 3 is a graph indicative of a typical gradient of refractiveindices, idealized, and representing the gradual transition from 100%SiO₂ to 100% SnO₂.

FIG. 4 is a section, somewhat simplified to facilitate the descriptionthereof of, of a novel apparatus of the type convenient for use in theprocess of the invention.

FIG. 5 illustrates the experimental measurement of the gradient inchemical composition of a silica-tin oxide gradient zone preparedaccording to the invention.

FIG. 6 shows an observed variation of the refractive index of theinitial deposit of SiO₂ --SnO₂ at the glass surface, as a function ofgas composition.

FIG. 4 illustrates a section of a lehr in a float glass line. Thestructure of the lehr itself is not shown for purposes of clarity. Thehot glass 10, e.g. about 500°-600° C., is carried on rollers 12, 14, and16 through the lehr. Between rollers 12 and 14 is positioned gas ductassembly 18 which comprises a gas inlet duct 20 and a gas outlet duct22. Between ducts 22 and 20 and separated therefrom by heat exchangingwall members 24 is a duct 25 forming means to carry a heat exchangefluid, which, in turn forms means to cool gas exhaust from duct 22 andto heat gas flowing through duct 20. The temperature of the heatexchange fluid is maintained at a sufficiently low temperature so thatcoating does not take place on the surface of the inlet duct.

Gas entering inlet 20 travels through a slit-like opening 28, thencealong a reaction zone formed by the top surface 30 of duct assembly 18and the lower surface of glass sheet 10. Upon reaching a secondslit-like opening 32, the remaining gas is exhausted through duct 22. Itis during the passage of the gas along the lower surface of glass sheet10 that a gradient coating is formed by the selective depletion of oneof the reactants at different points along the length of the depositionzone between rollers 12 and 14.

In the apparatus of FIG. 4 a second gas duct assembly 38 is used tocomplete the deposition of a coating, e.g. by adding a fluoride-dopedtin oxide coating to the pre-deposited gradient coating. Again, it isconvenient to have gas enter the upstream port 28a and leave thedownstream port 32a.

The ducting is suitably formed of corrosion resistant steel alloys andcomprises a jacket 50 of thermal insulation.

ILLUSTRATIVE EXAMPLES OF THE INVENTION

In this application and accompanying drawings there is shown anddescribed a preferred embodiment of the invention and suggested variousalternatives and modifications thereof, but it is to be understood thatthese are not intended to be exhaustive and that other changes andmodifications can be made within the scope of the invention. Thesesuggestions herein are selected and included for purposes ofillustration in order that others skilled in the art will more fullyunderstand the invention and the principles thereof and will be able tomodify it and embody it in a variety of forms, each as may be bestsuited in the condition of a particular case.

EXAMPLE 1

Glass heated to about 580° C. is moved at a rate of 10 cm/sec across theapparatus shown in FIG. 4. The temperature of the gas inlet duct ismaintained at a temperature of about 300° C., by blowing appropriatelyheated or cooled air through the temperature control duct. The firstdeposition region reached by the glass is supplied with a gas mixture ofthe following composition (in mole percent):

    ______________________________________                                        1,1,2,2 tetramethyldisilane                                                                           0.7%                                                  tetramethyltin          1.4%                                                  bromotrifluoromethane   2.0%                                                  dry air                 balance                                               ______________________________________                                    

The second deposition region is supplied with a gas mixture of thefollowing composition (in mole percent):

    ______________________________________                                        tetramethyltin         1.6%                                                   bromotrifluoromethane  3.0%                                                   dry air                balance                                                ______________________________________                                    

The flow rates of these gas mixtures are adjusted so that the averageduration of contact between a given element of the gas mixture and theglass surface is about 0.2 seconds.

The resulting coated glass is color-neutral in appearance, in reflecteddaylight. It has a visible reflectivity of 15%, and no visible haze. Theinfrared reflectivity is 90% at a 10 micron wavelength. The electricalresistance is measured to be 5 ohms per square. The coating is about 0.5microns thick.

EXAMPLE 2

The decomposition described in Example 1 is repeated, the onlydifference being the composition of the gas mixture supplied to thefirst deposition region:

    ______________________________________                                        1,2 dimethyldisilane  0.4%                                                    1,1,2 trimethyldisilane                                                                             0.3%                                                    1,1,2,2 tetramethyldisilane                                                                         about 0.02%                                             tetramethyltin        1.5%                                                    bromotrifluoromethane 2.0%                                                    dry air               balance                                                 ______________________________________                                    

The properties of the resulting product are indistinguishable from thoseof Example 1.

Samples of these coated glasses have been subject to Auger chemicalanalysis of the coating composition along with ion sputter-etching toreveal their chemical composition versus thickness. FIG. 5 shows theresulting chemical composition profile of the deposit over the region inwhich it varies. Near the glass surface the deposit is mainly silicondioxide, with about one silicon atom out of eight being replaced by tin.As the distance away from the glass surface increases, the tinconcentration increases and the silicon concentration decreases, so thatby distances greater than 0.18 micron from the glass surface, thedeposit becomes tin oxide, with about 1.5 percent of the oxygen replacedby the fluorine. Using FIG. 3, the silicon-tin composition profile isconverted to a refractive index versus distance profile, which is alsoplotted in FIG. 5. These results confirm the ability of the disclosedprocess to produce the desired variation of refractive index through thethickness of the deposited film.

EXAMPLE 3

A tin oxide coating is placed on a glass substrate at differentthicknesses (the glass substrate is first coated with an ultra-thin filmof silicon dioxide to provide an amorphous, haze-inhibiting surface.)

    ______________________________________                                        Thickness of Tin Oxide                                                                          Iridescence Visibility                                      ______________________________________                                        0.3 micron        strong                                                      0.6 micron        distinct, but weaker                                        0.9 micron        barely detectable except in                                                    fluorescent light                                          1.3 micron        weak, even in fluorescent light                             ______________________________________                                    

The latter two materials are not aesthetically objectionable forarchitectural use, confirming the visual color saturation scale used toevaluate the designs.

In order to provide the most effective suppression of iridescent color,it is desirable that the refractive index of the initial deposit matchclosely that of the glass substrate, preferably to within ±0.04, or morepreferably to within ±0.02 refractive index units. In order to achievethis match, one varies the parameters of the deposition, particularlythe ratio of tin to silicon atoms in the inlet gas. As an example ofsuch variation, FIG. 6 shows the variation of refractive index in theinitial deposit from tetramethyltin plus 1,1,2,2 tetramethyldisilane gasmixtures, as a function of gas composition. The other parameters forthese depositions were fixed as in Example 1. FIG. 6 shows, for example,that an initial deposit of refractive index 1.52 (appropriate to matchusual window glass refractive indices) is produced by a gas compositionof equal numbers of silicon and tin atoms. Matching to 1.52±0.02 isachieved when the gas composition is kept between 47 and 52 atomicpercent of tin. While these exact numbers may differ somewhat in otherconditions of deposition such as other temperatures or other compounds,it is a matter of routine experimentation to establish calibrationcurves such as FIG. 6, in order to produce a suitable match ofrefractive indices between the substrate and the initially depositedcoating composition.

It is to be noted that the reflection of light from the surface of thecoated products of Example 3 is about 16 to 17%, i.e. about 10% higherthan that from the coated glass in Examples 1 and 2 which do have agraded undercoat according to the invention.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which mightbe said to fall therebetween.

What is claimed is:
 1. A process for continuous coating of a substratewith a film formed of reactive components of a gas mixture in whichproperties of the film vary continuously through the thickness of saidfilm, said process comprising the steps of(a) flowing said gas mixturethrough a flow path defined by a reaction zone for said reactivecomponents, said reaction zone contiguous to and bounded by a surface ofthe substrate to be coated, (b) moving said substrate surface throughsaid reaction zone in a direction parallel to said flowing gas mixture,(c) depositing, preferentially, a reaction product derived from morereactive components of said mixture on said surface of said substrateexposed earlier to said gas mixture, thereby preferentially depletingsaid mixture of said more reactive components, (d) depositing,preferentially, a reaction product derived from less reactive componentsof said mixture on said surface of said substrate exposed later to saiddepleted gas mixture, thus (e) providing a product in which propertiesof said film vary continuously in the composition through the thicknessof said film, as said substrate emerges from said reaction zone.
 2. Aprocess as in claim 1 in which said substrate is composed of glass.
 3. Aprocess as in claim 1 in which said reaction products are produced byreaction of said gases induced by heat from said substrate.
 4. A procssas in claim 1 in which refractive index varies continuously from bottomto top through said coating.
 5. A process as in claim 4 in which saidgas mixture includes volatile silicon and tin compounds and an oxidizinggas.
 6. A process as in claim 5 in which said gas mixture includes atleast one partially alkylated polysilane, an organotin vapor, and anoxidizing gas.
 7. A process as in claim 6 in which said gas mixturecontains at least one methyldisilane and also tetramethyltin.
 8. Aprocess as in claim 7 in which said gas mixture contains 1,1,2,2tetramethyldisilane; 1,1,2 trimethyldisilane; 1,2 dimethyldisilane ormixtures thereof.
 9. A process as in claim 2 in which refractive indexvaries continuously from bottom to top through said coating.
 10. Aprocess as in claim 2 in which said gas mixture includes volatilesilicon and tin compounds and an oxidizing gas.
 11. A process as inclaim 2 in which said gas mixture includes at least one partiallyalkylated polysilane, an organotin vapor, and an oxidizing gas.
 12. Aprocess as in claim 2 in which said gas mixture contains at least onemethyldisilane and tetramethyltin.
 13. A process as in claim 2 in whichsaid gas mixture contains 1,1,2,2 tetramethyldisilane, HMe₂ SiSiMe₂ H;1,1,2 trimethyldisilane H₂ MeSiSiMe₂ H; 1,2 dimethyldisilane H₂MeSiSiMeH₂ or mixtures thereof.
 14. A process as in claim 2 in whichsaid reaction products are produced by reaction of said gases induced byheat from said substrate.
 15. A process as in claim 3 in whichrefractive index varies continuously from bottom to top through saidcoating.
 16. A process as defined in claim 10 wherein the proportions ofsaid reactive components are so selected to achieve a coatingcomposition proximate to said substrate of at least 60% SiO₂ and acoating composition most remote from said substrate of at least 95% tinoxide.
 17. A process for forming, on a substrate, a thin coating whichhas a progressively changing composition from a predominantly firstcoating composition nearer the substrate to a predominantly secondcoating composition more remote from the substrate, said processcomprising the steps of(1) introducing into a first end of a reactantchamber and out the other end of said chamber a mixture of(a) a firstreactant gas from which said first coating is formed, (b) a secondreactant gas from which said second coating compound is formed, and (c)a third gas which forms means to react with each of said first andsecond reactant gases to form said coating compounds,wherein said firstreactant gas reacts at a substantially different rate with said thirdgas, than does said second reactant gas, the different rates of reactionwith said third gas forming means to provide a difference in relativeconcentration of said reactant gases over the substrate as the thincoating is formed and to provide a changing ratio of said first andsecond coating compounds in said thin coating as said thin coating isformed on said substrate, (2) said introduction of gases being carriedout while continuously passing a substrate to be coated through saidreaction chamber from said first end of the chamber to said other end ofthe chamber.
 18. A process as defined in claim 1, 2, 3, 12, 13, 16 or 17wherein said reaction products are deposited at such a rate that saidchange in the coating composition is monotonic resulting in a gradualincrease of the refractive index of said coating as the thickness ofsaid coating on said substrate increases.
 19. A process as defined inclaim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 16, or 17 comprisingthe step of terminating said coating operation with an infra-redreflective overlay of tin oxide and wherein the total coating thicknessis from about 0.1 to 1.0 micron thick.
 20. A process as defined in claim1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 16, or 17 wherein saidcoating is of such a thickness that it forms means to suppress visibleiridescence, on the surface of a product of said process, as defined bya maximum Color Index value of about 12.